At the present time, a ruler 11 with a pattern of vertical lines 12 (having a certain line pairs per minimeter, lp/min) is often used as a target to adjust the focus of the lens in the scanner as shown in FIG. 1. However, there is a higher resolution in the horizontal direction but a lower resoluion in the vertical direction resulting from using the pattern of vertical lines of the ruler 11.
Please refer to FIG. 2 showing a theoretical diagram of the characteristic of a general lens. X specifies the resolution in the horizontal direction and Y specifies the resolution in the vertical direction. The ruler 11 provides an image to be received by a charge coupled device (CCD) for adjusting the focus of the lens in the scanner. During the process of adjusting the lens, the resolution of the image in the horizontal direction is not in accordance with that in the vertical direction. As shown in FIG. 2, the point A is the best point of the horizontal resolution. The modulation transport function (MTF) of the horizontal direction is about 0.35 but that of the vertical direction is only 0.04 (the MTF is proportional to the resolution). On the other hand, the point E is the preferable point of the vertical resolution, wherein the MTF in the horizontal direction is about 0.24 but that in the vertical direction is only 0.1. However, the desired point should be the point B rather than the extreme point A or E. The resolution of the lens is often focused on the point A by using the ruler 11 with a pattern of vertical lines.
FIG. 3 is a real diagram showing the characteristic of the lens. T specifies the best point 31 of the vertical resolution of the image of the subject P after providing an optical signal transmitted through the lens 30, C specifies the best point 32 of the vertical and horizontal resolutions, and S specifies the best point 33 of the horizontal resolution. If the ruler 11 with a pattern of the vertical lines is used as a target, only the horizontal resolution can be adjusted such that the resolution is usually adjusted on the point 33. Therefore, the horizontal resolution of the image is clear but the vertical resolution of the image is vague. The deviation of the horizontal resolution can be calibrated by taking advantage of the pattern of the vertical line so that the horizontal resolution can be adjusted in the best condition. However, the deviation of the vertical direction can not be calibrated. It seems impossible to adjust the resolution on the best point B as shown in FIG. 2.
In order to overcome this defect, the present invention provides a device for adjusting a resolution of an image-pickup apparatus. The focus of the lens can be fastly and exactly adjusted in the range between point C and D according to the characteristic of the lens as shown in FIG. 2 to obtain the best resolution (the point B). Thus, the efficiency for calibrating the optical apparatus can be improved to meet the requirements of high yield rate and low cost.